Pore-Scale Investigation for the Influence of Initial Water on CO2-CH4 Exchange in a Hydrate by CO2 Injection

In this work, a model is developed to account for diffusion-limited transport in the displacement process to study the effects of initial water on CO2-CH4 dynamic replacement in a hydrate in a real pore network. The influences of phase distributions on the fluid and reactions are also investigated. This model is validated by a previous experiment for the CO2-CH4 exchange process in methane hydrate. Our investigation results show that the exchange process with initial water has a faster carbon dioxide displacement, lower intensity methane hydrate dissociation, more pure carbon dioxide hydrate formation, almost the same mixed hydrate generation, and a lower exchange rate than that without initial water. The exchange reaction starts at the edges of hydrate distribution areas and progresses toward the inside. Both methane hydrate and mixed hydrate have lower saturation in the interior of hydrate distribution areas such as throats than that in the edges. Considerable pure carbon dioxide hydrate forms in the pores with initial water, resulting in unstable velocity and large pressure losses, which is harmful to the CO2- CH4 exchange process in the hydrate-bearing sediment.